refactor(metadata): invert state handler dependency

[hubcio]

Split define_state! into define_state! (struct + wrapper) and collect_handlers! (enum, Command, dispatch, Absorb). Each command type now implements StateHandler<S> for the state it mutates, moving handler logic from a central match block onto the command.

[codecov]

Codecov Report

❌ Patch coverage is 0.65789% with 302 lines in your changes missing coverage. Please review.
✅ Project coverage is 69.01%. Comparing base (85d73ca) to head (cde70cc).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
core/metadata/src/stm/stream.rs	0.00%	149 Missing ⚠️
core/metadata/src/stm/user.rs	0.00%	87 Missing ⚠️
core/metadata/src/stm/consumer_group.rs	0.00%	48 Missing ⚠️
core/metadata/src/stm/mod.rs	10.00%	17 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #2705      +/-   ##
============================================
+ Coverage     67.88%   69.01%   +1.12%     
============================================
  Files           734      571     -163     
  Lines         59374    55540    -3834     
  Branches      55787    55540     -247     
============================================
- Hits          40309    38330    -1979     
+ Misses        17108    15337    -1771     
+ Partials       1957     1873      -84     

Flag	Coverage Δ
java	?
rust	69.01% <0.65%> (-0.02%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
core/metadata/src/stm/mod.rs	41.79% <10.00%> (-4.12%)	⬇️
core/metadata/src/stm/consumer_group.rs	0.00% <0.00%> (ø)
core/metadata/src/stm/user.rs	0.00% <0.00%> (ø)
core/metadata/src/stm/stream.rs	0.00% <0.00%> (ø)

... and 168 files with indirect coverage changes

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